The present invention relates to an arrangement and a rotor for screening of pulp. The arrangement and rotor according to the invention are especially suitable for the screening of chemical and mechanical pulps, i.e. fiber suspensions of the wood processing industry.
This application claims priority to international patent application PCT/F100/01014 filed Nov. 22, 2000, and to Finnish application U990488 filed Nov. 29, 1999.
Naturally, prior art knows several devices used for screening fiber suspension. Reference is here made to solutions according to U.S. Pat. Nos. 5,000,842, 5,224,603 and 5,547,083 which are meant for screening fiber suspensions at a relatively high consistency, which in the field of screening means a consistency of about 2.5-5%. Said consistency is so high, that in order to maintain the screenability of the pulp, special characteristics and construction are required from the pulsation member, i.e. the rotor, to prevent the pulp from forming excessively large and strong fiber accumulations in the screening area. E. g., in the above patents the rotor is essentially cylindrical and on the surface of the cylinder there are so-called bulges arranged according to a certain configuration for maintaining the turbulence level and pulsation of the suspension.
During the years, the screen comprising said rotor has proved to be a reliable and advantageous apparatus, but in some situations the consistency of the suspension in the screening area between the screen drum and the rotor tends to rise so high that said bulges are not capable of increasing the turbulence level high enough to maintain optimum screening efficiency. One solution for the problem is to apply dilution of the suspension in the screening area.
Accordingly, prior art knows several various solutions for diluting the pulp between the cylindrical rotor and the also cylindrical screen drum. As an example of these solutions e.g. U.S. Pat. No. 4,234,417 may be mentioned, in which the surface of the rotor cylinder is provided with blades extending to the whole height of the rotor. On the trailing side of these blades, when looking in the rotating direction of the rotor, there are outlet ports via which dilution liquid is fed in the suspension. Said outlet ports lead inside the rotor, where there are three annular chambers arranged one upon the other so that dilution liquid is fed into each chamber from outside the screen.
Said construction has both advantages and disadvantages. The only advantage worth mentioning is that the dilution liquid is fed specifically via the rotor, whereby it would not be necessary to guide it near to the screening surface. Nevertheless, when performing according to the patent, i.e. when bringing the dilution liquid via the blades of the rotor onto the back surface of the blades, to the area of intense suction, there is a great risk that the dilution liquid is passed onto the screening surface and therethrough quickly further to the accept side without actually diluting the pulp in the screening area. A second disadvantage worth mentioning is the complexity of the construction, as e.g. making the hole of FIG. 5 requires two opposite drillings in the blade and additionally one drilling in the surface of the rotor cylinder.
CA patent 1007576 discloses an another example of a rotary pulp screening device in which dilution water is directed against the screen. There is provided a rotary pulp screening apparatus including a housing with a stock inlet chamber, stock screening chamber, a cylindrical screen and a rotor in the form of a truncated cone having an upper portion and a lower portion. The improvement comprises a circumferential dividing ring extending around the wall of the lower portion of the cone, means for fastening the ring to the wall of the lower portion and a plurality of dilution water nozzle""s positioned in the lower portion adapted to direct water against the screen. The dividing ring may be moved up or down the lower portion of the rotor, or it may be welded in a predeterminated position. The dividing ring generally permits the effect of the dilution water to be localized in one area because it stops the water from rising upwards. The impeller rotor, being in the form of a truncated cone, may be provided with a series of blades or with foils, whereas the present invention relates to an essentially cylindrical rotor with turbulence-generating members such as so-called bulges.
Said problems have been solved by the arrangement and rotor according to the present invention, the characteristic features of which are disclosed in the appended claims.